Understanding the structure and properties of geological formations often reduces the cost of drilling wells for oil and gas exploration. Measurements are typically performed in a borehole (i.e., downhole measurements) in order to attain this understanding. To obtain such measurements, a variety of sensors and mounting configurations may be used.
For example, resistivity tools are widely used in oil field exploration applications for determining zones in a formation that may contain hydrocarbons. Accurate determination of the formation resistivity is useful in assessing the feasibility of production for a particular well. However, resistivity measured by the tool is affected by factors other than the formation resistivity. For example, the presence of the borehole itself, as well as invasion zones, may change the measurement values. A borehole correction can be applied to offset the effect of the borehole. However, invasion effects are often harder to resolve because invaded formation resistivity values may approach those of the non-invaded formation, which makes the invasion radius difficult to determine. For example, when the invasion resistivity is equal to the formation resistivity, inversion may determine the invasion radius to lie between zero to infinity—either of which might be equally correct.